In exhaust gas of vehicles, carbon monoxide, hydrocarbons, and nitrogen oxides are included as harmful materials. Among them, nitrogen oxides cause environmental problems such as photochemical smog and acid rain, as well as diseases in humans. Therefore, there is a desire to improve engines and develop a post-treatment technique of exhaust gas.
The most effective technique for removing nitrogen oxides is a selective catalytic reduction (SCR) method. This method has been developed according to a reducing agent such as ammonia (NH3), urea, hydrocarbons (HC), and the like, and various catalysts. Among them, the selective catalytic reduction method that uses ammonia (NH3) as the reducing agent is most effective in removing nitrogen oxides in a fixed source such as a power plant and an incinerator. However, in order to apply it to vehicles that are moving sources, there is a problem of storage, transport, and use of ammonia, so a method for using urea that is capable of being easily decomposed to ammonia by heat decomposition and a hydration reaction as a reducing agent is desired.
Meanwhile, as the catalyst that is applied to the selective catalyst reduction method, zeolite-based catalysts such as copper (Cu)/zeolite having excellent functions may be used. Such catalysts are generally required to maintain their catalyst characteristics even at a high temperature in order to process an exhaust gas having a high temperature.
The above information disclosed in this Background section is only for enhancement of understanding of the present disclosure and therefore it may contain information that does not form part of the prior art that is already known in this field to a person of ordinary skill in the art.